It is a small and simple organic molecule. A typical protein is far larger than this, and it takes trillions of proteins to make a mycobacterial cell about the size of the pores in a Sawyer filter.

FD&C Blue 1 is sometimes called erioglaucine:

It is also a small, simple molecule. The tinyness of these, relative to the pores in the Sawyer filter, cannot be overstated. Trillions of these could be lined up end-to-end across one pore in a Sawyer filter.

Jama, your results stump me. Is the dye viscous (syrupy)? I can't fathom why these dyes would be retained by a filter with 0.1 micron pores and then backwashed out again. Hopefully we will learn something interesting from this. It would be very handy to have a simple dye test to confirm the integrity of these filters.

Jerry, that is emphatically not the meaning of that. That is a wavelength of light absorbed by the dye. It has nothing to do with the size of the molecule.

Molecules are not things that you could almost see if you squinted hard enough. They are unfathomably tiny, and no molecule will ever get physically lodged in a pore of a 0.1 micron filter. Or a 0.01 micron filter. Or a 0.00001 micron filter. A mycobacterium about the size of the pore in a Sawyer filter is made of TRILLIONS of molecules, most of which are proteins far larger than FD&C Red 40 or Blue 1.

Dye molecules are not physically lodging in the pores of Jama's filter. This still might be a good test for the ability of a filter to retain pathogens (I don't know), but retention of the dye occurs by some other mechanism.

But maybe size of molecule is related to wavelength absorbed? That was the only distance mentioned in the wikipedia article. That's a lot of atoms in that molecule but I don't think it's anywhere near 0.6 micron or 0.2 for that matter. I am not a chemist : )

I'm skeptical about this test but it would be nice if something like that worked.

No, the size of a molecule is not related to wavelength absorbed. I was a chemistry major in college until I changed over to a double major in microbiology and molecular biology with a chemistry minor. We can forget about the issue of size. I promise. The dye molecules are far, far, far too small to be physically trapped by the filter.

Jama's result doesn't make sense to me. I don't see how a filter of this kind could retain a soluble dye. So I had to do my own test. I have only used my Sawyer filter once, in summer, and I also have a new (never used) Sawyer inline filter (with a 0.1 micron filter component). And my wife informed me that she has some DecAcake brand food coloring. The red is FD&C Red 40 and the blue is FD&C Blue 1.

So, just now, I did a test. By adding water with the food coloring directly to the back of each filter, then blowing gently through them, I made blue and red water come out the clean end. The filters do not retain the dye, and I wouldn't expect them to. I don't know why Jama's results are different. The ingredients on the blue dye package are water, propylene glycol, Blue 1, citric acid, sodium benzoate, and propylparaben. The red dye ingredients are the same except for the dye component (Red 40).

I'm more into physics - in order to absorb a wavelength the front and back surface have to be like 1/4 of the wavelength apart so reflections off the back surface cancel reflections off the front surface. But for the number of atoms in Blue #1, it's more like 1 nm big so nowhere close to filter size.

I too tried a test with some unnamed dye that said it included blue #1 - passed right through filter.

Perhaps I can help a bit. Let's break 'filters' up into three classes:mechanicalactivated carbonion exchange

The mechanical ones include all the usual suspects, including the osmotic ones like the Sawyers. They can filter down to some relatively large diameter. In the case of the Sawyer 0.01 micron filter, they can even filter out viruses, but viruses are absolutley huge compared to simple dye molecules.

The activated carbon ones do some mechnical filtering, but they also do some chemical or molecular filtering. The surface of the *activated* carbon can bind some molecules or ions down so they are stripped out of the water. Many kitchen filters are like this: they can strip chlorine and ammonia ions out of the water but they can't block viruses - or even bacteria sometimes. Good for taste, no good for 'purifying' to EPA standards.

Ion exchange filters go further than the carbon ones. They feature a special 'plastic' matrix which attracts and binds most anything. They can filter out all the bugs and wogs plus many chemicals. The only one I know of is the First Need filter - pity it is so heavy. It would take out dye molecules.

Now some mechnical filters do include a core of activated carbon, so they can filter out bacteria and some chemicals. This can improve the taste of the water. But they cannot normally filter out viruses because they are too small for the mechanical part and they don't bond to the carbon.

Could an activated carbon filter remove some dye molecules? Doubtless. But you can saturate the carbon.

Of course the point isn't to filter out the dye, like with activated carbon, but to determine if your filter is still filtering out particles that are 0.02 micron or whatever - to verify that it would filter parasites and maybe bacteria.

How do they test filters to verify they meet their specs of filtering down to 0.02 microns or whatever they're rated for?

I have tested filters (including a Sawyer Squeeze) in my lab by filtering water spiked with fluorescent microbeads (red ones 2.8 microns in diameter and green ones 6 microns in diameter) then looking at the filtrate under the microscope to confirm that there were no beads. This test will establish whether the filter will remove protozoan parasites (Crypto, Giardia, Toxoplasma, etc.), but I would need much smaller beads (about 0.3 microns) to test whether the filter will retain pathogenic bacteria.

Also, it occurred to me: although the dye test obviously isn't useful for determining whether a filter will remove pathogens, it might be a good way to guess whether a filter will remove (or reduce) tannins.

I have access to these beads because I already use them for my research. Testing a filter doesn't use very many, and there are tens of millions in a small bottle, but the bottle costs something like $900 if I remember right. And you need access to an epifluorescent scope with DAPI and Texas red filters to see them. It is a very easy test to do (it takes about an hour) if you have the equipment. But most people don't.

I don't know if there is a practical way to test the integrity of a filter at home. A colorimetric test would be much more practical for most people than a test requiring microscopy. One problem is sensitivity. Even if a solid colorimetric reagent could be acquired as particles of the right size, a very small number of particles would have to produce a color change in the filtrate for the test to be meaningful. The minimum infectious innoculum for bacterial pathogens is hundreds to thousands of organisms, but for protozoans as few as one organism (ie, for Toxoplasma) can cause an infection. A single particle in the filtrate is easy to see under the microscope, but I can't imagine a colorimetric test that would be so sensitive that a single particle would provide the required color change.

Okay, I'm an idiot. All things were NOT equal. I am really sorry using your time and launching this discussion (albeit interesting and scientific) when my experiment wasn't run properly.

It turns out when I ran the blue dye experiment with two Sawyer filters last week, the new filter started out full of clean, clear water trapped inside the filter. I had just taken it out of the package and back flushed it. I did not evacuate this clear water before taking my sample. The other filter (my old one) had sat around for a couple weeks in storage and was more or less dried out. I didn't evacuate it either, but the first water out was the blue water I had just put in.

I also used small clear juice glasses to collect the results, and because they didn't hold much water, the new filter didn't even get to the blue dyed water before I quit filling the glass. Thus it had only clean water. When I ran the experiment tonight and evacuated the pre-existing water in both filters, both ran blue.

Bottom line ... blue dye does not filter out in my Sawyer filters and probably doesn't in yours either, and I'm back to square one. Again, so sorry to have misled anyone, especially those people who never get to page 2 of this discussion.

I'm going to try very hard to redeem myself by posting my just-finished cuben fiber gravity-feed sling for my Sawyer filter. The sling weighs between 0.4 and 0.5 oz. Suspended from a tree branch, it will filter a liter of water in a few minutes. I made it because sometimes I would like the filter to work unattended while I do something else, like unpack my lunch. Top bag holds the dirty water bladder to which the Sawyer filter is attached. The cone channels the drip into the soda bottle seated in the bottom bag. The entire unit hangs from a tree branch. I hate to add any weight to the design, but it might be nice to have a small cup or pocket to hold the bottle cap while the bottle is filling, as well as a net pre-filter, so the entire water system is in one place.

I've spent a lifetime doing science research. The first real lesson you learn in this game is that you eventually throw away the first couple of experiments, because they were where you were learning what it is you are really trying to measure. It happens with utter reliability.

Remember the credo for the experimentalist: 'If we knew what we were doing, it wouldn't be research.'

What you HAVE done is to cross check your own work properly. Excellent. We all learn.

Jama,I love your inquisitiveness. And from your gear list, I can tell you have studied a lot. I like how you went down to the detail--- even a hairband! And you understand your calorie intake. That takes several trips to figure out. Wow.

Anyway, the dye test only works with First Need. For over a quarter century no one has duplicated their element that filters pathogens, viruses, chemicals, and tannin (for us psychological people who like to drink clear water instead of brown water).

How do you know other filters works? This is an area where the Steripen also does well; you know right off the bat if it’s working. If there is no visible blue light--- then you know there’s no UV light.

“...but it would be great if there were some sort of .1+ micron test kit. One of you DIY-ers should come up with that product. I'd buy it.”

Yes, you would be rich with that invention --- if it could be kept low cost. Or start a lab and offer a service.